Mongo group by wihtout repeating

Here is a query


  stage[] = [
            '$project' => [
                '_id'          => 1,
                'name'     =>1,
               'booking_id'=>1
            ]
        ];
stage[] = [
            '$lookup' => [
                'from'         => 'booking',
                'localField'   => 'booking_id',
                'foreignField' => '_id',
                'as'           => 'booking'
            ]
        ];
        stage[] = [
            '$unwind' => [
                'path'                       => '$booking',
                'preserveNullAndEmptyArrays' => false
            ]
        ];

        stage[] = [
            '$group' => [
                '_id'          => '$_id',
                'name'     => ['$first' => '$name'],
            ]
        ];

My data is properly grouped and projected but i need only data with a booking to it. so i joined booking table and unwinded to get rid of empty records. but this made me to group again. is there any way to group with out i have to type each field i needed again and again? in the above example code i have to write 'name' => ['$first' => '$name'],
normally i have 50+ such fields. i have a feeling that there is a better way. will $$ROOT works. i didnt understood how to use it with groupby. please help